The present invention relates to gas turbine engines, and more particularly, to a control system for such engines.
Of importance in the control of a gas turbine engine used as a helicopter powerplant is the need to govern the speed of the power turbine. Typically, the speed of the power turbine is desired to be constant under substantially all conditions. Such constant speed governing must be maintained in even the most severe flight maneuvers. Deviation from constant speed, or isochronous operation, is sometimes referred to as "rotor droop." It is known that such "droop" can be reduced by increasing the bandwidth of the speed control through an increase in the loop gain of the speed control system thereby making the control system more responsive to deviations from the desired constant speed. However, it is also well known that this loop gain cannot be increased without causing the control system to become unstable at frequencies indicative of the mechanical resonances of the main rotor system and the tail rotor system. Both of these resonances tend to be very lightly damped as a result of coupling large inertial masses (rotors) to the power turbine through shafts and springs which provide very little damping. Consequently, it is a conventional practice to limit the loop gain of the speed control to a level which keeps the mechanical resonances adequately suppressed and to accept a corresponding compromise in the magnitude of the "rotor droop" which will be tolerated. It is also commonly known that the amplitude and frequency of the main and tail rotor resonances are different on different airframes and that the amplitude of the resonances can vary with flight velocity.
It is, therefore, an object of the present invention to provide an improved control system for a gas turbine engine.
Another object of the present invenion is to provide such a control system for use in combination with a powerplant of a helicopter.
Another object of the present invention is to provide such a control system in which mechanical resonances of the main and tail rotors are suppressed, permitting increased control system loop gains with stable control system operation.
Another object of the present invention is to provide such a control system which is adaptable to different airframes.
Another object of the present invention is to provide estimator means for estimating the main rotor speed error in a helicopter.